1. Field of the Invention
The subject invention generally relates to a variable effort power steering system for a vehicle, and more specifically to a method of determining a direction of a power boost applied to a power cylinder of the power steering system.
2. Description of the Prior Art
Power steering systems typically include a pump, a control valve, a power cylinder, and a reservoir, all interconnected by a plurality of fluid lines. The pump draws a hydraulic fluid from the reservoir and supplies the hydraulic fluid to the control valve. The power cylinder includes a first fluid chamber and a second fluid chamber. During steering maneuvers, the control valve selectively directs the fluid to one of the first fluid chamber via a first fluid line or the second fluid chamber via a second fluid line to provide the power boost (a power assist force) to the power cylinder to assist a driver in the steering maneuver. Alternatively, if no steering maneuver is currently being performed, the control valve directs an equal pressure of the hydraulic fluid to both the first and second fluid chambers of the power cylinder.
Variable effort power steering systems alter a required steering effort to steer a hand wheel (steering wheel) of the vehicle as a road speed of the vehicle changes. The variable effort steering systems increase the required steering effort at higher road speeds to increase a road feel, and lower the required steering effort at lower road speeds to help a driver turn the hand wheel.
The control valve in a variable effort power steering system regulates the power boost applied to the power cylinder by varying the pressure of the hydraulic fluid directed into either the first or second fluid chambers, thereby affecting the required steering effort as is well known in the art. In the variable effort power steering systems, a controller determines the required steering effort based on the road speed of the vehicle, and signals the control valve to increase or decrease the pressure of the hydraulic fluid directed to the first and second fluid chambers of the power cylinder to increase or decrease the power boost provided by the power cylinder. United States Published Patent Application 2006/0249367A1, Ser. No. 11/122,337, published on Nov. 9, 2006, and assigned to Delphi Technologies, Inc., the assignee of the this application, discloses such a variable effort steering system.
In order to operate the variable effort power steering system, the controller must receive information relating to the magnitude of the power boost being applied to the first and second fluid chambers in the power cylinder and to the direction of the power boost, i.e., which one of the first and second fluid chambers the power boost is being applied. The controller utilizes this information to calculate a change in the power boost applied to the first and second fluid chambers of the power cylinder to vary the required steering effort. Typically, a first pressure sensor is in fluid communication with the first fluid line connecting the control valve and the first fluid chamber for sensing the power boost being applied to the first fluid chamber, and a second pressure sensor is in fluid communication with the second fluid line connecting the control valve and the second fluid chamber for sensing the power boost being applied to the second fluid chamber. The first pressure sensor and the second pressure sensor send their respective sensed pressures in the first and second fluid chambers to the controller. The controller compares the pressure from each of the first and second pressure sensors to determine in which direction the power boost is being applied. In other words, if the first pressure sensor in fluid communication with the first fluid chamber senses a fluid pressure that is higher than the second pressure sensor in fluid communication with the second fluid chamber, than the controller determines that the control valve is directing the power boost toward the first fluid chamber at the pressure sensed by the first pressure sensor. Likewise, if the first pressure sensor senses a pressure less than the second pressure sensor, than the controller determines that the control valve is directing the power boost toward the second fluid chamber at the pressure sensed by the second pressure sensor.
It would be desirable to eliminate one of the pressure sensors in the variable effort power steering system, while still maintaining the capability to calculate the direction and magnitude of the power boost applied to the first and second fluid chambers. Eliminating one of the pressure sensors decreases the overall cost of the power steering system.